ON THE MEAN HEIGHT OF THE BAROMETER. 279 



6. The leading column in Table I. contains a set of annual barometrical means 

 taken (with the exception of the last) from those I have already published in the 

 ' Climate of London.' These are calculated from the Tables for each month in the 

 ordinary way, and not on the solar years. I have given them as they stand in that 

 work, though in the years from 1815 to 1817 they ought possibly to be higher by a 

 tenth of an inch, from the too high placing the scale in those years ; but this (with 

 other like inaccuracies which may be hereafter found and rectified) I do not consider 

 as affecting much the proportions found among the results in any given year. In cal- 

 culating the set of averages on periods of nine years, placed in the last column of this 

 Table, I have, however, to prevent discrepancies, added this tenth of an inch upon 

 each of the three years. 



7. The fifth column of Table II. contains the barometrical mean, calculated upon 

 the whole period of solar years, which, in the four preceding columns, are averaged 

 under the respective lunar positions of north, south, &c. The sixth column of this 

 Table shows a mean founded on a direct average of the four results placed under 

 these heads. I have noticed some features of the variation at the foot of the Tables. 

 I shall proceed now to state some general results, of course as to the barometer alone. 

 The effects on the mean temperature and rain must for the present be left unnoticed. 



The barometrical mean in our climate is depressed {on an average of years) by the 

 moon's position in south declination. 



In every one of these averages upon periods of nine years, in Table II., the mean 

 under south is lower than that under north declination ; the diflference being in some 

 cases between six and seven hundredths of an inch : and it is larger on the averages 

 in the fore-part than on those in the latter part of the series. 



The mean under south declination is also lower than either of the other three ; with 

 exception of the four latter averages, in which it exceeds a little that of the position 

 "going south." 



This depression is gradual: it commences with the moon in full north declination, 

 and proceeds through her remaining positions to the time when she again crosses the 

 equator to return north ; at which season the whole weight that had been abstracted 

 is suddenly restored — this of course must be understood of the small differences la 

 the mean here treated. There will be found, in the observations employed, an 

 abundance of particular cases of variation which contradict such a rule, but the com- 

 pensations, it appears, cover these in its favour. 



We have here, I think, evidence of a great tidal wave or swell in the atmosphere, 

 caused by the moon's attraction, preceding her in her approach to us, and following 

 slowly as she departs from these latitudes. Were the atmosphere a calm fluid ocean 

 of air of uniform temperature, this tide would be manifested with as great regularity 

 as are those of the ocean of waters. But the currents, uniformly kept up by the 

 sun's varying influence, effectually prevent this, and so complicate the problem. 



There is also manifest in the lunar influence a gradation of effects, which is here 



